New York — As the nation continues to grapple with the emotionally charged aftermath of the grand jury decisions in the police killings of two black men, Eric Garner and Michael Brown, perhaps no political leader has more at stake than New York Mayor Bill de Blasio.

The longstanding and nationwide issues surrounding policing and minority communities, after all, are Mr. de Blasio’s signature issue. As he railed against “stop and frisk” and other aggressive policing as a candidate last year, his message resonated, catapulting him from a little-known local leader into one of the nation’s progressive chief executives – on a global stage in America’s largest metropolis.

But now that the issue has gripped the nation’s consciousness, de Blasio could be facing one of the most important tests of leadership, one that could not only determine the success of his tenure as mayor, but also influence the direction of the nation as he attempts to reshape both the city and the New York Police Department, the largest and arguably most influential force in the United States.

“He is on something of a precipice of what could either be his moment, or what could easily go horribly wrong,” says Jeanne Zaino, professor of political campaign management at New York University’s School of Professional Studies. “Either way, I think this is really an important moment for him.”

On Thursday, President Obamacommended the mayor for his emotional speech following the grand jury’s decision in the Garner case. The grand jury declined to indict a police officer for a headlock takedown that contributed to the man’s death on a Staten Island sidewalk.

Indeed, many observers have noted a change in the New York mayor since the grand jury decision on Wednesday, suggesting, as the well-worn expressions goes, that he has finally “found his voice” this year, after a number of missteps.

“The relationship between the police and the community has to change,” an impassioned de Blasio said at the New York Police Academy Thursday. “The way we go about policing has to change. It has to change in this city; it has to change in this country. I am fundamentally convinced it will change.”

The mayor – who has drawn some criticism from his supporters for not specifically speaking out against the grand jury decision, as even many conservatives have – outlined the NYPD’s new annual training program. That program, announced after Mr. Garner’s death, will emphasize that officers treat all citizens with respect and with regard for their safety, even during arrests.

Like Mr. Obama, many others have noted de Blasio’s deeply personal and emotional 20-minute speech on Wednesday, after the grand jury decision.

“When I turned on the television and saw de Blasio speaking, the first thing that I noticed was that he didn’t have a prepared text, which stunned me,” says Kenneth Sherrill, professor emeritus of political science at Hunter College in Manhattan. “And I thought to myself, how can you negotiate this mine field? There’s so many mistakes you can make, so many wrong words you can inadvertently choose.”

De Blasio spoke of the “unspeakable pain” of Garner’s father and told New Yorkers that the issue is profoundly personal for his own family because of the dangers he perceives for his biracial son, Dante.

The mayor also suggested that the current problems are deep-seated – rooted in not just decades, but centuries of racism in the country.

But these are the issues that have put the de Blasio administration on a political precipice all year. New York City police officers bristle at any suggestion of racism, and they continue to mistrust the mayor – and they vehemently resent how close he is to the Rev. Al Sharpton.

After being elected, de Blasio in fact has muted his criticisms of the NYPD, upon which his administration must rely. So far, crime has continued to fall to record lows this year, and any uptick could imperil the mayor’s entire progressive agenda.

The difficult relationship between the mayor and the NYPD can be seen in comments from Ed Mullins, president of the Sergeants Benevolent Association and a 30-year veteran of the NYPD.

“The mayor has opportunities here to really build a brand, to really build a reputation for himself,” he says. “But it cannot be a one-sided tale of two cities, a city that he thinks [represents] his beef with inequality.”

“He has to speak for all people, and his statement [Wednesday], in response to what took place, and how he had to talk to his son, I really think is divisive,” Sergeant Mullins continues.

Although the reactions to the grand jury decision in New York have been much more united than those after the decision in Missouri, challenges for de Blasio’s leadership loom, observers note.

“It’s awfully hard for anyone in public office, a mayor or any chief executive, to sympathize with protesters,” says Professor Zaino. “And yet if anything goes awry, if there’s any violence, you’re going to get the blame for that.”